The invention relates generally to crashworthy seats for a moving vehicle, in particular, to energy attenuating crew seats for a helicopter.
It is known to use an energy attenuation system to connect a helicopter crew seat to a supporting structure. The energy attenuating system may include vertically extending wires, affixed at opposite ends to the support structure and passing through a three-roller trolley attached to the seat. The wire is sized so that it will bend and unbend over the rollers, and allow the seat to move downward, or stroke, when a specified force acts upon the seat in a downward direction. This specified force is selected to be less than an inertia force acting upon the occupant of the seat during a crash of the helicopter which is likely to cause fatal damage to the occupant. When a crash occurs, a seat will start to stroke downward when the inertia force acting upon it in the downward direction increases to the specified force. As the seat strokes downward, energy is absorbed by the bending and unbending of the wires over the rollers of the trolley, and inertia force acting upon the seat occupant is maintained at a constant level, even through the surrounding structure may experience inertia forces of much higher levels.
The inertia force applied to the seats which is necessary to cause stroking during a crash of the helicopter is determined by multiplying the weight of the occupant and seat by the acceleration or G level. Thus, since the specified force required to initiate stroking of the seat is a constant of the energy attenuation system, it is seen that a light-weight seat occupant will experience a higher G level than a heavy-weight seat occupant. Also, for the same crash conditions, the seat will stroke a greater distance when occupied by a heavy-weight occupant than it will when occupied by a light-weight occupant. Thus, the specified force required to initiate stroking of the seat must be selected on the basis of the weight of the lightest occupant expected to use the seat, and the seat stroke distance must be selected on the basis of the weight of the heaviest occupant to use the seat.
As a result of having to use the weight of the lightest occupant to select the force necessary to initiate stroking of the seat, the G level experienced by a heavy seat occupant will be much lower than that required to protect him against fatal damage, and a seat will stroke through a much greater distance than would otherwise be required. For example, in a known crashworthy crew seat for a military helicopter, when the seat is occupied by a light-weight occupant wearing only tropical clothing, the seat will begin to stroke when the acceleration rises to approximately 22 G, and will stroke for approximately 4 and 1/2 inches, under specified crash conditions. Under the same specified crash conditions, when the seat is occupied by a heavy occupant wearing survival gear, body armour, and arctic clothing, so that a total weight of the heavier occupant is 100 pounds greater than the above-mentioned lighter occupant, the seat will begin to stroke when the acceleration reaches approximately 14.3 G, and the seat will stroke approximately 11.2 inches.
In the past, the energy attenuator systems for crashworthy crew seats in a helicopter have been set to initiate stroking of the seat at one specific inertia force level to protect the lightest expected occupant; consequently, these known seats must be designed to provide a minimum of 12 inches of clearance space between the lower surface of the seat pan when adjusted to its lowest position and the basic aircraft floor structure to provide the minimum stroking distance required when the seat is occupied by the heaviest expected occupant. This requirement imposes severe restrictions to the structural designs of new aircraft, and, in many cases, prohibits the successful retrofit of a crashworthy seat in an existing aircraft.